1. Field of the Invention
The present invention relates to a tunable Bessel low pass filter which is applied to a multi-rate duobinary generation system of an optical communication system, and more particularly to a tunable high-order Bessel low pass filter which is capable of, when the data rate of an application system is changed, controlling its filtering characteristics most appropriately to the changed data rate, thereby extending its application range and enhancing performance and efficiency of the application system.
2. Description of the Related Art
In general, an optical communication system comprises an optical transmission system including an optical transmitter for converting an electrical signal into an optical signal, an optical fiber cable for transmitting the optical signal from the optical transmitter, a repeater for amplifying and transmitting the optical signal from the optical fiber cable in the middle of its transmission by the cable, and an optical receiver for converting the optical signal amplified and transmitted by the repeater back into an electrical signal. Such an optical communication system of the 10 Gbps class has various data rates, including 12.5 Gbps, based on standardization of optical transmission networks, resulting in a need for electronic devices for generation of the corresponding signals to have tunable ranges.
FIG. 1 is a block diagram showing the configuration of a general duobinary generation system.
With reference to FIG. 1, the general duobinary generation system comprises a data signal or pulse pattern generator 111 (referred to hereinafter as a ‘data signal generator’) for generating data or a pulse pattern, a precoder 112 for exclusive-ORing an electrical data signal from the data signal generator 111 and a 1 bit-delayed version of the data signal to code the data signal, a fixed Bessel low pass filter 113 for low pass filtering an output electrical signal from the precoder 112, an amplifier 114 for amplifying an output electrical signal from the fixed Bessel low pass filter 113 at a predetermined gain, and an optical modulator 116 for modulating light generated by a laser diode 115 into an optical duobinary signal on the basis of an output signal from the amplifier 114.
On the other hand, the above-mentioned duobinary generation system may transmit data at multiple rates, such as 10 Gbps, 12.5 Gbps, etc., according to transmission environments or data types. At this time, the data transmission must be made with no signal loss irrespective of a change in data rate. However, since a conventional fixed fifth-order Bessel low pass filter, such as the fixed Bessel low pass filter 113, is implemented in an appropriate manner to a specific data rate, it must be replaced with a different one upon occurrence of a change in data rate, as will hereinafter be described with reference to FIG. 2.
FIG. 2 is a block diagram showing the configuration of the conventional fixed fifth-order Bessel low pass filter 113.
With reference to FIG. 2, the conventional fixed fifth-order Bessel low pass filter 113 functions to filter an input signal Sin to pass a low-frequency component thereof and cut off a high-frequency component thereof containing an unnecessary noise component. To this end, the fixed Bessel low pass filter 113 includes a plurality of fixed capacitors C1–C3 and a plurality of fixed inductors L1 and L2.
This conventional fixed fifth-order Bessel low pass filter has a disadvantage in that it is not operable adaptively to a change in data rate because it is designed and used in an appropriate manner to a specific data rate. For example, in the case where the low pass filter filters a lower-rate signal under condition of having higher passband characteristics, it distorts the signal, resulting in signal loss. Similarly, in the case of filtering a higher-rate signal under condition of having lower passband characteristics, the low pass filter distorts the signal, resulting in signal loss.
That is, the conventional fixed fifth-order Bessel low pass filter has a ‘−3 dB’ cut-off frequency corresponding to the specific data rate. For this reason, provided that the data rate is changed, the conventional fixed fifth-order Bessel low pass filter must be replaced with a different one.
In other words, the conventional fixed fifth-order Bessel low pass filter can perform an appropriate filtering function required by the system with respect to one specific data rate, but cannot perform it due to signal distortion, etc. with respect to a different data rate. For this reason, there is a problem in that the conventional fixed fifth-order Bessel low pass filter must be replaced with a different one appropriate to the different data rate, or the system to which it is applied must be replaced with a different one.